This invention relates generally to electrical measuring and testing and, more particularly, to product measuring and testing involving temperature cycling.
Environmental test chambers are widely used to test electrical products such as printed circuit boards and other electrical/electronic products which include a printed circuit board as a component part. Such testing involves monitoring certain electrical operating characteristics of the product while it is undergoing extreme changes in temperature. Temperature cycling over a range of +125xc2x0 C. to xe2x88x9265xc2x0 C. (about +255xc2x0 F. to xe2x88x9285xc2x0 F.) is not uncommon and is often accompanied by extreme changes in humidity and/or by vibrating the product under test. In the industry, testing of this type is often referred to as xe2x80x9cstress testing.xe2x80x9d
A primary reason that product stress testing is undertaken is to identify particular products (within a larger group of products) which exhibit characteristics evidencing probable premature failure. And such testing is intended to cull out those products which actually fail during test. In the vernacular of the industry, such products are said to exhibit xe2x80x9cinfant mortality.xe2x80x9d Those products which do not exhibit infant mortality are much more reliable in the automotive, aircraft, military or other application in which they are used. U.S. Pat. No. 3,656,058 (Leathers); U.S. Pat. No. 4,683,424 (Cutright et al.); U.S. Pat. No. 4,949,031 (Szasz et al.) and U.S. Pat. No. 5,021,732 (Fuoco et al.) all disclose apparatus used for environmental stress testing.
Designers of environmental test chambers must deal with a number of engineering considerations. One is the rapidity with which the temperature in the product-containing chamber can be changed. In a test chamber having refrigeration and heating systems of a particular size, the rate at which the temperature can be changed is, in significant part, a function of the mass (e.g., the mass of the products and product carriers) in the chamber. This is so because the heat absorbed by the chamber contents and the heat which must be removed therefrom is a function of such mass.
A household refrigerator is a good analogy of the foregoing. For a particular refrigerator, two pounds of foodstuffs in the refrigerator are more quickly cooled to a particular temperature than twenty pounds of foodstuffs. And as a corollary, cooling twenty pounds of foodstuffs to a particular temperature within a stated time requires a larger refrigeration unit than is required to cool two pounds of foodstuffs to the same temperature within the same time.
Another engineering consideration involves the equipment used to monitor the electrical operating characteristics of the products under test. Such equipment is temperature sensitive and must be maintained nominally at room ambient conditions. In other words, such equipment should not be in the chamber with the products undergoing test.
Still another engineering consideration is whether to configure the test chamber for batch-process or continuous- process testing. The apparatus of above-noted Szasz et al. patent is for batch testing in that a number of products are placed on a pallet which is inserted into the chamber. All the products on the pallet undergo test simultaneously and after such test is completed, the pallet and its xe2x80x9cbatchxe2x80x9d of products is removed and another pallet loaded with products to be tested is inserted.
On the other hand, the vibration chamber disclosed in U.S. Pat. No. 5,226,326 (Polen et al.) may be referred to as a type of continuous testing arrangement. Such chamber uses a conveyor having spaced pairs of rollers to grasp respective edges of flatwise-oriented printed circuit boards to be tested. Similarly, U.S. Pat. No. 5,397,998 (Soeno et al.) discloses several different arrangements of a conveyor and products to be tested carried atop such conveyor. In one arrangement, feeder apparatus along the conveyor supply electric power to the products during burn-in and the xe2x80x9cburned-inxe2x80x9d product are then tested after exiting at the end of the conveyor.
While these earlier arrangements are thought to have been generally satisfactory for their intended uses, they are not without disadvantages for some types of applications. For example, the conveying arrangements shown in the Soeno et al. patent apparently do not permit instrumented product testing while the product is moving through the chamber. In other words, such instrumented testing is carried out after the product leaves the burnin chamber. The xe2x80x9cfailure modexe2x80x9d characteristics exhibited by the products while in the burn-in chamber and after they leave such chamber may differ markedly.
Yet another disadvantage of the conveying arrangements of the Soeno et al. patent is that the mass of the conveyor (as well as that of the product to be tested) is in the burn-in chamber. Chamber temperature changes can be accomplished and maintained only by adding heat to or removing heat from the conveyor components.
Still another disadvantage of prior art arrangements is that they seemingly have not appreciated how to configure test chambers so that the size and capacity thereof can be selected or changed to suit a particular application. For example, the arrangement shown in FIG. 5 of the Polen et al. apparently has a fixed length which cannot be changed. At least, there is no suggestion to the contrary.
A new environmental test apparatus which addresses certain shortcomings of earlier apparatus would be an important advance in the art.
It is an object of the invention to provide an environmental test apparatus which overcomes certain problems and shortcomings of the prior art.
Another object of the invention is to provide an environmental test apparatus which is useful for stress testing electrical and electronic products.
Another object of the invention is to provide an environmental test apparatus which permits simultaneous product electrical testing and temperature stress testing.
Still another object of the invention is to provide an environmental test apparatus which minimizes the mass contained in the thermal chamber.
Another object of the invention is to provide an environmental test apparatus which implements continuous process testing.
Another object of the invention is to provide an environmental test apparatus by which continuous process testing may be carried out while maintaining the test instrumentation substantially at room ambient conditions.
Yet another object of the invention is to provide an environmental test apparatus which may be xe2x80x9ccustom-configuredxe2x80x9d for any one of a variety of applications. How these and other objects are accomplished will become apparent from the following descriptions and from the drawings.
The invention involves an environmental test apparatus of the type having a thermal chamber for stress testing electronic products. Such apparatus includes a carrier for supporting the products in the chamber and instrument-type test equipment for monitoring product characteristics during stress testing.
The improved apparatus has an auxiliary chamber and a flexible partition interposed between the thermal and auxiliary chambers. The carrier is in the thermal chamber (where the temperature environment is made intentionally harsh) and the test equipment is in the auxiliary chamber which is nominally at room ambient temperature and humidity. A highly preferred embodiment of the carrier includes a flat, sheet-like product such as a feedthrough card having attached thereto a fixture for releasably mounting an electronic product, e.g., a printed circuit board, on the carrier.
The carrier includes first and second opposed, parallel carrier surfaces and the partition includes first and second partition members bearing against the first and second carrier surfaces, respectively. A preferred partition member is a strip-mounted brush with carbon-bearing bristles for reducing static electricity. Separate partition members are mounted at either side of the carrier and resiliently bear against a respective carrier surface to prevent significant air transfer between the chambers.
It is particularly desirable to prevent warmer, more humid air in the auxiliary chamber from migrating or transferring to the thermal chamber since such transfer speeds the rate at which frost accumulates in the thermal chamber. In a highly preferred apparatus, there are plural partitions, one being a primary partition. One (and preferably two) auxiliary partitions are spaced from one another and from the primary partition and are interposed between the primary partition and the auxiliary chamber.
While the partition members perform acceptably to prevent air transfer between chambers and, particularly, to prevent such transfer from the auxiliary chamber to the thermal chamber, it is preferred to take yet additional measures to inhibit air transfer. The spaced-apart partitions define a flow path between them. Dry air is caused to flow along the flow path, thereby substantially preventing moisture from migrating from the auxiliary chamber to the thermal chamber.
The new apparatus has yet other features which facilitate movement of the carrier along the thermal chamber and which facilitate product monitoring during temperature stress testing. The apparatus includes a support platform in the auxiliary chamber. The carrier is attached to the support platform and extends downwardly through the partition. An exemplary support platform is a square or rectangular slab made of DELRIN(trademark) plastic or the like.
A conveying mechanism is also in the auxiliary chamber and the platform is in driven engagement with the conveying mechanism to cause movement of the platform along the auxiliary chamber. A specific conveying mechanism has two spaced-apart sections, each such section comprising an endless belt supported by rollers and, typically, driven by one roller. The spaced-apart edges of the platform rest on respective conveying sections.
It is also highly desirable to be able to electrically operate the products under stress test as they move along through the apparatus. Thus, the auxiliary chamber contains a power bus embodied as two parallel, spaced-apart rails. The support platform includes collector shoes in electrically-conductive relationship to the power bus, thereby providing power to products mounted on the carrier. In one preferred embodiment, a test board or a board computer is in the auxiliary chamber, is supported by the platform, moves with such platform and is electrically connected to both the power bus and to the product(s) under test.
To better accommodate several product carriers with their associated platforms, the thermal chamber is horizontally elongate and an air plenum is coextensive with the thermal chamber. The air plenum is isolated from the thermal chamber along most of the length of both but there are openings at either extreme end of the plenum and thermal chamber so that the plenum is in air flow communication with such chamber. A motor-driven blower, e.g., a centrifugal blower, urges air along the plenum and the thermal chamber. A refrigeration evaporator is in the air plenum so that the air being circulated along the plenum and the thermal chamber may be cooled for stress test purposes.
The new apparatus is thoughtfully configured so that it can be applied in any one of several application requirements. The apparatus includes a plurality of modules attached to one another. The modules cooperatively function in xe2x80x9cbuilding blockxe2x80x9d fashion in that a portion of the thermal chamber is in each of the modules. Stated in other words, the modules can be attached to one another end to end to form an apparatus of the desired length and product testing capacity. The plurality of modules includes at least one module having a door mounted for movement between an open position for placing products in the thermal chamber and a closed position for stress testing the products. Typically, a door module is placed at either end of the assembly of modules. And, conveniently, each module has levelling feet, thereby permitting the modules to be aligned with one another.
Because the new apparatus has thermal and auxiliary chambers which are isolated from one another and because the auxiliary chamber is essentially always at room ambient temperature, the chambers preferably have differing wall configurations. The auxiliary chamber has a plurality of first walls, the thermal chamber has a plurality of second walls and each of the second walls is substantially thicker than each of the first walls. And each of the second walls includes an insulation layer making up at least one-half of the wall thickness.
Other details of the new apparatus are set forth in the following detailed description and in the drawings.